Sodium/Bicarbonate Cotransporter NBCn1/Slc4a7 Inhibits NH4Cl-mediated Inward Current in Xenopus Oocytes

• Authors: Soojung Lee, Emory University; Inyeong Choi, Emory University
• Journal Title: Experimental Physiology
• Volume: Volume 96, Number 8
• Publisher: Wiley: 12 months | 2011-08, Pages 745-755
• Type of Work: Article | Post-print: After Peer Review
• Publisher DOI: http://doi.org/10.1113/expphysiol.2011.057844
• Subjects: Biology, Physiology
• Keywords: bicarbonate; Xenopus; voltage-clamp

Abstract

The electroneutral Na/HCO3 cotransporter NBCn1 (SLC4A7) contributes to intracellular pH maintenance and transepithelial HCO3− movement. In this study, we expressed NBCn1 in Xenopus oocytes and examined the effect of NBCn1 on oocyte NH4+ transport by analyzing changes in membrane potential, current, and intracellular pH mediated by NH4Cl. In the presence of HCO3−/CO2, applying NH4Cl (20 mM) produced intracellular acidification of oocytes. The acidification was faster in oocytes expressing NBCn1 than in control oocytes injected with water. However, NH4Cl-mediated membrane depolarization was smaller in oocytes expressing NBCn1. In HCO3−/CO2-free solution, NH4Cl produced a smaller inward current in NBCn1-expressing oocytes (56% inhibition by 20 mM NH4Cl; measured at −60 mV), while minimally affecting intracellular acidification. The inhibition of the current by NBCn1 was unaffected when BaCl2 replaced KCl. Current-voltage relationships showed a positive and nearly linear relationship between NH4Cl-mediated current and voltage, which was markedly reduced by NBCn1. Large basal currents (before NH4Cl exposure) were produced in NBCn1-expressing oocytes due to the previously characterized channel-like activity of NBCn1. Inhibiting this channel-like activity by Na+ removal abolished NBCn1’s inhibitory effect on NH4Cl-mediated currents. The currents were progressively reduced over 72–120 h after NBCn1 cRNA injection, during which the channel-like activity was high. These results indicate that NBCn1 by its Na/HCO3 cotransport activity stimulates NH4+ transport, while reducing NH4+ conductance by its channel-like activity.